Shared storage systems have recently been developed significantly. The shared storage systems are storage systems in which a plurality of storage devices are shared by a plurality of host servers. Each of the shared storage systems generally incorporates a management server for managing the entire system.
A shared storage system utilizes logical units, The logical units are also called logical volumes or logical disks, and have logical storage areas. An arbitrary storage position in a logical storage area is designated by a virtual address. The logical storage area is associated with at least a part of a physical storage area in at least one of the plurality of storage devices. The management server holds address management information (e.g., address management tables), for the respective logical units in the shared storage system, in order to manage the correspondence between virtual addresses and real addresses in the storage devices for the respective logical units.
Assume here that a host server in the shared storage system accesses a target storage position in a logical unit mounted (provided) therein. In this case, the host server interrogates the management server for the address (i.e., real address) of the storage position in a storage device, which is associated with the virtual address of the target storage position, and the storage device (more specifically, the identifier of the storage device). Namely, when accessing a target storage position in a logical unit, the host server accesses the management server in order to ascertain a storage position in a storage device, which is associated with the target storage position. Such access to the management server also occurs when each of the other host servers in the shared storage system accesses a target storage position in a logical unit.
As described above, when accessing a target storage position in a logical unit provided in a respective one of the plurality of host servers in the shared storage system, the respective one must access the management server. In this shared storage system, concentration of access on the management server by the host servers is regarded as a bottleneck in enhancing the performance of the entire system. In other words, even if, for example, the number of host servers or the number (or storage capacity) of the storage devices is increased in the shared storage system, further concentration of access on the management server will occur, which makes it difficult to achieve system performance that covers such an increase.
To overcome the concentration of access on the management server, the following structure may be contrived. Namely, a plurality of host servers in the shared storage system may hold respective copies of address management information stored in the management server. In such a structure, a respective one of the plurality of host servers can acquire a real address associated with the virtual address of a target storage position, based on the copy of the address management information, held by the respective one. This means that each host server can access the target storage position without referring to the management server.
In the above structure, however, if the address management information has been updated in accordance with updating of data by, for example, a first host server, it is difficult to make the address management information coincide between the first host server and the other host servers.